1. Field of the Invention
The present invention relates to amorphous, spherical calcium carbonate particles, a production method thereof and use therefor.
2. Related Technology
Calcium carbonate, CaCO3, is a calcium salt of carbonic acid, which today is used in many areas of daily life. For example, it serves particularly as an additive or modifier in paper, paints, plastics, inks, adhesives and pharmaceuticals. In plastics, calcium carbonate is used primarily as a functional filler agent, as a replacement for the relatively expensive polymer, to improve mechanical or optical properties and/or to make processing easier.
Calcium carbonate occurs naturally in various phases. In principle, a distinction is made between the hydrous and the anhydrous phases. Calcite, vaterite and aragonite are structures with no stored water content and have the same stoichiometry (polymorphism). Two hydrated crystalline phases of calcium carbonate also exist: the monohydrate and a hexahydrate (ikaite).
Besides the crystalline forms, amorphous calcium carbonate (ACC) is also known. ACC is a metastable phase that occurs with variable water content and in which the atoms do not form ordered structures but rather an irregular pattern, and so only have a short-range order, not a long-range order. ACC is unstable and transforms into calcite at temperatures above 300° C. This process is accelerated by the presence of water, and crystallization takes place at lower temperatures.
ACC can be prepared on the basis of many different starting materials and under many different reaction conditions, for example from a calcium chloride solution that is reacted with sodium bicarbonate in the presence of magnesium ions, with ammonium carbonate or sodium carbonate, or by hydrolysis of dimethyl carbonate in a calcium chloride solution.
The latter option is discussed notably in the dissertation by M. Faatz, Kontrollierte Fällung von amorphem Calcium carbonate durch homogene Carbonatfreisetzung Johannes Gutenberg-Universität Mainz 2005, in which two synthesis variants are examined in detail:
In the first variant, 0.001 mol calcium chloride in 100 ml aqueous solution is reacted with 0.001 mol dimethyl carbonate in the presence of 0.002 mol sodium hydroxide. Alternatively, 0.001 mol calcium chloride in 100 ml aqueous solution is caused to react with 0.005 mol dimethyl carbonate in the presence of 0.010 mol sodium hydroxide. The precipitates obtained in each case are isolated and dried, although more precise information regarding drying is not given. Both methods result in more or less spherical amorphous calcium carbonate particles with a residual stored water content of 0.4 mol to 0.6 mol relative to 1 mol calcium carbonate, or 7% by weight to 10% by weight water relative to total weight.
In the course of this work, the suitability of ACC for use as a filling material in ultrahigh molecular weight polyethylene (UHMW-PE) was also studied, and in this context the particles were dispersed in situ in the growing polymer chains to avoid chain scissions. The filled polymers obtained in this way have a melting peak in the range from 137° C. to 139° C., which is lower than that of the pure UHMW-PE (146° C.)
A refinement of these spherical amorphous calcium carbonate particles is described in patent application WO 2008/122358, which disclosed spherical calcium carbonate particles with an mean particle diameter in the range from 0.05 μm to 2.0 μm and a water content (residual moisture at 105° C.) not exceeding 5.0% by weight, relative to the total weight thereof, and an improved property spectrum.
The calcium carbonate particles are obtained in aqueous solution by reacting calcium chloride with dialkyl carbonate in the presence of an alkali metal hydroxide, wherein the components are advantageously used in the following concentrations:
a) CaCl2: >10 mmol/l to 50 mmol/l;
b) Dialkyl carbonate: >10 mmol/l to 50 mmol/l;
c) Alkali metal hydroxide: 20 mmol/l to 100 mmol/l.
They are particularly suitable for use as additives or modifiers in paper, paints, plastics, inks, adhesives and pharmaceuticals.
However, the stability of the both the amorphous calcium carbonate particles obtained by Faatz and of the amorphous calcium carbonate particles of WO 2008/122358 is limited, since they are transformed into crystalline modifications over time. Accordingly, the stability of these particles in the reaction solution is of extremely short duration and is normally only in a range from a few minutes to a few hours. Therefore, in order to be able to use them as an additive or modifying agent, they must be processed further very quickly, which is very difficult to carry out, if not impossible, particularly on an industrial scale.
The stability of the isolated and dried calcium carbonate particles described by Faatz or in WO 2008/122358 is somewhat better, but it is still short-lived. From the technical point of view, therefore, calcium carbonate particles with improved long-term stability are desirable.
Furthermore, the known calcium carbonate particles have very high basicity, which makes them very difficult, if not impossible to used in many fields of application.
In addition, from European patent application EP 1 151 966 A1 a method is known for producing stable, lamellar calcitic calcium carbonate with a granulometric factor of about 0.5 to 1 and an mean particle diameter of about 0.5 to 1.5 μm and comprising the following steps:
calcium hydroxide in aqueous suspension is converted to basic calcium carbonate by introducing CO2 with an energy input of about 25 to 200 kW/m3,
one or more additives from the group consisting of organically substituted phosphonates, polycarboxylates or double-hydrophilic block copolymers in an amount of about 0.5 to 5% by weight relative to the calcium carbonate is/are stirred into the suspension as surfactants,
additional CO2 is introduced into the suspension relatively slowly besides the addition of the surfactants, and
the calcitic calcium carbonate formed thereby is separated by filtration.
However, no reference is made to amorphous calcium carbonate in this document.
The publication by N. Abdel-Aal, K. Sawada Inhibition of adhesion and precipitation of CaCO3 by aminopolyphosphonate Journal of Crystal Growth 256 (2003) 188-200, deals with the effect of ethylenediamine-N,N,N′,N′-tetrakis-methylenephosphonic acid (EDTMP) on the formation and conversion processes of calcium carbonate that clings to the surface and was precipitated in solution. In this context, the effect of EDTMP on the quantity, speeds and adhesion times was investigated for various EDTMP concentrations. The effect on these reactions of the calcium concentration in the solution was also studied.
For this purpose, a predetermined quantity of a calcium chloride solution was added rapidly to a sodium carbonate solution, wherein the latter already contained a predetermined quantity of EDTMP. When the two solutions were mixed, a precipitate formed immediately in each case.
QCM sensors were immersed in the reaction solution and subsequently analyzed using scanning tunneling microscopy and X-ray diffraction. Under these circumstances, the lowest EDTMP concentration (10−6 mol/l) yielded spherical calcite particles approximately 9 μm in size (FIG. 6). An EDTMP concentration of 10−5 mol/l yielded spherical calcite particles with a size of about 8 μm (FIG. 7). At the highest EDTMP concentration (10−4 mol/l), the adhesion of calcium carbonate to the QCM sensors was severely inhibited and a small number of needle-shaped aragonite crystals and calcite particles were obtained on the QCM sensors (FIG. 8).
The publication assumes that at least intermediately amorphous calcium carbonate is formed during the reaction under study. This is assumed to have a very small particle size and a rough surface (page 197, right column, lines 9-13). However, it is not possible to deduce any more accurate information about the size and shape of the amorphous calcium carbonate from this document.
The publication by R. Pairat, C. Sumeath Precipitation and Dissolution of Calcium—ATMP Precipitates for the Inhibition of Scale Formation in Porous Media Langmuir 1997, 13, 1791-1798, examines the precipitation reaction of amino trimethylene phosphonic acid (ATMP) and calcium. Crystalline or lamellar calcium carbonate or powdery, spherical amorphous calcium carbonate is formed depending on the precipitation conditions.
The publication by An-Wu Xu, Qiu Yu, Wen-Fei Dong, Markus Antonietti, Helmut Cölfen Stable Amorphous CaCO3 Microparticles with Hollow Spherical Superstructures Stabilized by Phytic-Acid Adv. Mater. 2005, 17, 2217-2221, describes the reaction of calcium chloride with ammonium carbonate in the presence of phytic acid. In this context, hollow calcium carbonate spheres with a size distribution in the range from 1 to 2.8 μm are formed from nanoscale amorphous calcium carbonate particles. The resulting hollow spheres are claimed to remain stable for longer than 14 days.